Alfalfa weevil survived winter, larvae threaten first-cutting hay

COLUMBIA, Mo. – “Alfalfa fields face a double hit by weevils this spring. There are more adult weevils than I’ve ever seen,” says Wayne Bailey.

In 29 years of scouting alfalfa fields, the University of Missouri Extension entomologist has seen lots of weevils.

His advice: Scout fields often. “Small weevils are easy to miss,” Bailey says. “A small pocket of them can expand rapidly and do lots of damage.”

There is a problem with alfalfa-weevil control this year. Cold weather lowers the effect of insecticides used to control them.

“With ground rigs, apply mixes with 20 gallons of water per acre,” Bailey says. “Ask aerial applicators to apply as much as they can carry.”

Some pilots prefer to drop rates to a half-gallon of water per acre. “That won’t give control against this population,” he says.

Bailey isn’t sure why so many adult alfalfa weevils are out laying eggs. He thinks it has to do with the extreme-cold weather last winter.

The perception was that below-zero weather kills overwintering adults. “The bugs didn’t read the book,” Bailey adds. “It seems they just dug deep in ground litter and hibernated.”

In normal years, the overwintering population stay active, laying eggs. Not this winter. The winter brood is out now, laying eggs. Those fresh-laid eggs have high survival rates. Winter-laid eggs often die before they can hatch in the spring.

The new brood of weevils will be laying eggs for weevils to emerge in May and June. Good control now will reduce that second generation.

The alfalfa faces a big threat, as every new female lays up to 500 eggs. Each egg produces a small larva that hides, and eats, in the growing tips of emerging alfalfa.

Once the tiny larvae move out of the leaf folds, they defoliate an entire plant. Larvae have a big appetite, Bailey says. Lost leaves reduce hay quantity and quality.

“Weevils cause a lot of damage in a short time.”

With the weevil population explosion, larvae are ripe for a fungal outbreak. That natural control turns the worms lemon yellow before they die. Even though larva may stay alive, once infected they stop eating.

The fatal fungus needs wet weather, which is in the forecast. But it also needs warmth, which is not in the forecast.

Bailey advises growers to prepare for spraying. And to hope for warm weather.

Another option is to cut near-mature alfalfa for hay.

But haying allows weevils to concentrate on regrowth from the alfalfa stubble. The worms nip new leaves as they emerge. Spraying probably will be needed after the first cutting for weevil control.

If regrowth is not protected, the stand thins and weeds grow to shade the alfalfa. At that point, weeds crowd out alfalfa.

Bailey gave his alfalfa-weevil alert during a weekly teleconference from the MU campus to regional specialists in the counties.

The specialists reported the best alfalfa growth was in southwestern Missouri, where the hay is near knee-high. Alfalfa growth slowed with cool, cloudy weather across the rest of the state.

“If you haven’t looked at your alfalfa, you better look,” Bailey says. “The hay crop may need help with weevil control. Natural control might not work this year.”

Bailey demonstrates techniques for scouting alfalfa for insects in a video at http://youtu.be/65v2YqSBcdg.